In my copending application, Ser. No. 149,539, above-identified, the latent electrostatic image is developed by electrophoresis of toner particles through a liquid carrier which is a non-toxic, light, paraffinic hydrocarbon. The freshly developed moist image is then transferred across an air gap to a carrier sheet. In the prior art, part of the carrier liquid in the non-image areas will be absorbed by the carrier sheet and must be dried, usually by heat. This evaporates hydrocarbons into the circumambient atmosphere, and the amount of evaporation permitted is strictly controlled by law. This reduces the speed at which the electrophotographic copying machine can be operated. A non-toxic, light, paraffinic hydrocarbon carrier liquid, such as ISOPAR-G (trademark of Exxon Corporation), is one of the aliphatic hydrocarbon liquids which I use in my composition. The contacting of a carrier sheet with the freshly developed image will induce smudging, smearing, or squashing of the developed image. This reduces the resolution. Then too, the charge of the toner particles is opposite to the charge of the latent electrostatic image. This arrangement is such, in the prior art, that the paper tends to stick to the photoconductive, or insulating, surface on which the image is developed. This produces difficulty in removing the carrier sheet bearing the developed image from the photoconductive surface. The usual carrier sheet is paper, and repetitive contact of paper with a moist developed image leaves paper fibers on the photoconductive surface. Since all of the developed image is rarely transferred to the carrier sheet, the paper fibers contaminate the developing liquid.
I have found, as pointed out in the copending applications, above-identified, that these disadvantages can be avoided by spacing the carrier sheet from the photoconductor to form a gap and causing the freshly developed image to negotiate the gap between the photoconductor and the carrier sheet by placing a charge on the back of the carrier sheet by means of a corona or the like.
In my copending application, Ser. No. 149,539, now U.S. Pat. No. 4,364,661 I describe the method of transferring freshly liquid-developed images across a gap. I disclose methods of forming a gap by providing the carrier sheet with protuberances formed on the carrier sheet which prevent the contact of the major area of the carrier sheet with the freshly developed image by deforming the sheet or otherwise forming protuberances thereon. In the copending application of Benzion Landa and E. Paul Charlap, Ser. No. 249,336, now U.S. Pat. No. 4,378,422 there is disclosed another means of carrying out my method. We there provide spacing particles to form the desired gap between the substrate bearing the freshly developed electrostatic image by positioning them on the developed image or by forming spacing protuberances on the photoconductive, or insulating, surface on which the latent electrostatic image is formed.
I have discovered that I may accomplish substantially the same result by another means--namely, by disseminating spacer particles adapted to prevent the carrier sheet from contacting the freshly developed image in the developing composition of this invention so that these particles are spaced throughout the developed image and the background areas, thus forming the desired gap over which the transfer of the developed latent electrostatic image occurs.
In order to remove excess carrier liquid from the photoconductor so as to reduce the danger of wetting the carrier sheet to which the developed image is to be transferred, I use a reverse roller which shears the excess developing liquid from the surface of the photoconductor, after the image has been developed, without disturbing the developed image. This is described in Hayashi et al U.S. Pat. No. 3,907,423.
In order to prevent the removal of a large number of spacer particles from the surface of the photoconductor in the non-image areas where they are not held by the charge of the electrostatic image, I bias the reverse roller. This charge should be of the opposite polarity as the polarity of the charge on the toner particles, since this will reduce the deposition of toner on the background areas and prevent the background areas from being gray. If the spacer particles do not have a surface charge which is the same as the charge of the toner particles, the toner particles will tend to deposit on the spacer particles. This will produce black dots on the background areas where the spacer particles contact the carrier sheet. It will be appreciated that, to perform their function in spacing the carrier sheet from the surface of the photoconductor, the spacer particles are interposed between the surface of the photoconductor and the carrier sheet. Furthermore, if the spacer particles acquired a charge opposite to the charge of the toner particles, not only would black dots be created in the non-image areas, but the spacer particles would become covered with toner particles and settle to form a hard, non-dispersible mass.
1. Field of the Invention
The invention relates to an improved composition for developing latent electrostatic images by liquid toning, in which a gap is formed across which transfer takes place.
2. Description of the Prior Art
Machida, in U.S. Pat. No. 3,915,874, discloses a liquid developer for use in developing a latent electrostatic image and then transferring it to a carrier sheet by contact between the carrier sheet and the developed image in which resolution is increased by preventing crushing of the toner particles forming the developed image. He does this by suspending fine particles which are harder than the toner particles throughout the liquid carrier which is any of the known aliphatic hydrocarbon liquids used in dielectric liquid-carried toner particles forming developing liquids of the prior art. The fine anti-crushing particles employed by Machida are inorganic materials, such as glass beads, zinc oxide, titanium dioxide, silica, and the like. The average fine inorganic particles have a diameter of from 1.mu. to 15.mu.. Machida erects a signpost to the art against the instant invention by pointing out that, above a 15.mu. diameter of the hard, fine particles, there is an increase in white spots which destroy the image and the resolution. There is no disclosure of using spacer particles of such large size as to prevent contact between the carrier sheet and the developed image by forming a gap. The "white spots" mentioned by Machida are "holidays" in the transferred image. The "fine" particles of Machida are equal to or smaller in diameter than the toner particles, so that there is contact between the developed image and the carrier sheet to which the image is being transferred.